Conventional aircraft incorporate highly complex, integrated airframe and powerplant control systems. Common sensor signal access, system data exchange and other communication between a flight control computer and a digital electronic control (DEC) of a gas turbine engine is advantageously utilized to reduce pilot workload while maintaining safe, efficient aircraft operation. Since system faults may occasionally occur which disrupt the communication links, critical operational failure modes are typically anticipated and redundant hardware and control functions provided to ensure a high level of system reliability. Where such redundancy is either too costly to provide or not warranted due to the remote likelihood of occurrence of a particular failure scenario, system backup methods are routinely employed. For example, loss of a noncritical sensor signal may cause the DEC to operate the engine in a fail-safe mode, sacrificing fuel efficiency to maintain minimum safe power output.
In the case of a turbofan engine, where thrust is provided by a rotating fan, operational performance is often quantified as a function of flight conditions. Depending on the location of the aircraft in the flight envelope, fan speed may be modulated to maintain a minimum guaranteed thrust level. Hot ambient temperature, high altitude cruising requires higher fan speed to achieve the same thrust as cold, low altitude cruising due to the difference in ambient air density. The correct power scheduling in a DEC of such an engine, usually in terms of low pressure rotor or fan speed, N1, conventionally requires input of altitude, Mn and ambient temperature signals to accurately set fan thrust.
A conventional system may employ a digital transmission link between the aircraft flight computer and the DEC, as some of these signals are typically available as part of aircraft instrumentation; however, to meet safety requirements in the event of loss of communication with the flight computer, the DEC must be capable of maintaining a minimum thrust level without any aircraft input signals. While the DEC usually has available redundant engine sensor signals for inlet total free stream ambient air temperature, T1, and static free stream ambient pressure, P0, used to determine altitude, an aircraft Mn signal is generally not independently available at the engine.
In the event the aircraft Mn signal is lost, the DEC could set fan speed generally higher than necessary to ensure meeting minimum required thrust for the worst anticipated operating conditions. This mode of operation, while providing sufficient operating margin, typically results in higher core and fan rotor speeds, hotter turbine inlet temperatures, increased fuel consumption and a concomitant reduction in engine component life. Further, there may exist regions of the flight envelope, especially in high performance aircraft, where engine limits inhibit such aggressive operating parameters, due for example to stall margin boundaries.
Alternatively, upon loss of the aircraft Mn signal, the DEC could generate an independent Mn value, based upon independent sensor signal inputs. A conventional method of deriving Mn at a given point in a gas flow entails measuring the total gas pressure and the static gas pressure at the point of interest. Since Mn is a function of the ratio of total to static pressures, the Mn value may be readily computed if the appropriate pressure signals are available. While such a direct measurement method could be employed, the cost, weight and complexity associated with the addition of dedicated pressure sensors used solely for backup modes of operation is typically not warranted. Further, this method produces a value for the Mn of the flow proximate the sensors, which may be located in a fan bypass duct, for example, or other location remote from aircraft ambient, the Mn value of interest. Placement of sensors external to the engine configuration is generally beyond the purview of the engine manufacturer and would simply replicate existing aircraft sensors. Yet further, additional wiring harnesses and connectors required to provide remote sensor signals to the DEC comprise additional failure nodes which compromise overall system reliability.
Another method of deriving aircraft Mn is to use engine sensors, including an inlet pressure sensor, in combination with a multivariable engine operating parameter schedule to calculate the total to static pressure ratio, P1/P0, proximate the engine inlet. In a particular exemplary control system, engine sensor inputs comprise fan speed, N1, inlet temperature, T1, inlet duct static pressure, PS12, and ambient static pressure, P0. T1 is used in combination with N1 to generate a value of corrected fan speed, N1R. Based on inputs of N1R and the measured ratio PS12/P0, a value for inlet total pressure, P1, may be generated since the relationship between N1R, PS12 and P1 is quantifiable for a fixed inlet area. P1 may then be used in a ratio with measured P0 to generate Mn as described hereinabove.
Use of any such method necessitates a sensor set which includes an inlet pressure sensor. Many engines do not have the requisite sensor signals available, especially PS12; therefore, a single use PS12 sensor would have to be incorporated, along with the associated cost, weight and reliability penalties associated therewith. Alternatively, compressor discharge static pressure (CDP) sensors are widely used in modern engines to support a variety of control functions, including preventing combustor case overpressure, maintaining compressor surge margin and setting minimum flight idle conditions. Advantageous use of the available CDP signal, also referred to as PS3, in a Mn synthesis method would obviate the need to provide a separate inlet sensor in engines of this type, as well as the associated wiring harness and connectors.